Glass fabric produced with zero-twist yarn

ABSTRACT

The present invention relates to a glass fabric produced with zero-wist yarn, its use an the manufacture of printed circuits and in numerous other industrial applications, as well as to a process for the manufacture thereof.

The present invention relates to a glass fabric produced with zero-twistyarn, its use in the manufacture of printed circuits and in numerousother industrial applications, as well as to a process for themanufacture hereof.

BACKGROUND OF THE INVENTION

Glass fabric is used today with success in the most varied applications.The most important include those relating to advanced structuralcomposites for the aeronautical and ship-building industries anddielectric composites for the electrical and electronics industry.

In particular, the Applicant is interested in the manufacture of glassfabrics for use in the electrical and electronics industry, preferablythe construction of laminates for printed circuits and the manufactureof fabrics and gauzes used as a reinforcement in numerous industrialapplications.

Printed circuits nowadays represent the most widespread and effectivesupport medium for electronic equipment, ensuring both the mechanicalanchoring and electrical connection of the associated components, and itis not possible to foresee any other connecting systems capable ofreplacing them in the medium or short term.

Consequently the laminates required for their construction have animportant role and it is expected that their production volume willsteadily increase for many years to come.

The printed circuits and the laminates used for their construction arerequired to have increasingly superior characteristics as a result ofthe evolution of microelectronics technology which enables anincreasingly large number of logic circuits to be concentrated on thechips or on the physical support components.

The miniaturization of electronics devices has given rise to variousassembly problems including the increased concentration of tracks on theprinted circuits and hence the need to make them thinner.

As a result the amount of copper adhering to the laminate must bereduced. Under the impetus of this requirement, the thickness of thecopper lamina of the printed circuits has been reduced, in a few years,from 70 microns to 35 and 18 microns, and the tendency is to reduce itstill further to 12, or even 9 microns.

This innovative process has not radically affected the productioncriteria for laminates, but has made it necessary to use glass fabricswhich are much more refined than those currently used.

In particular, textile manufacturers are being required to find, withincreasing urgency, solutions to the problems associated with thesurface of the fabrics which must always be flat and smooth and as faras possible free from loose strands and textile defects in general. Infact any filaments of the glass yarn present on the surface of thelaminate penetrate into the copper during the pressing process, whilerougher surfaces diminish the adhesion of the copper lamina to thesupport base of the epoxy-glass laminate.

State of the Art

The requisites for the supporting glass fabrics in laminates for printedcircuits are closely linked to the performance required of the latter.

The laminate which is currently most widespread--called FR--4--has thecharacteristics shown in the table below:

                  TABLE                                                           ______________________________________                                        Technical characteristics of the laminate FR-4                                ______________________________________                                               ε                                                                              4                                                                    Tg (°C.)                                                                        140                                                                  HPCT (hours)                                                                           2                                                                    VO (pass/fail)                                                                         Pass                                                                 C.sub.u (N/mm)                                                                         2                                                                    α.sub.z (ppm/°K.)                                                         200                                                                  P(t) (μm)                                                                           5                                                                    D stab (ppm)                                                                           400                                                           ______________________________________                                         ε = Dielectric constant                                               Tg = Glass transition temperature                                             HPCT = High Pressure Cooker Test                                              VO = Selfextinguishing characteristics (in accordance with UL 94)             C.sub.u = Bonding strength (copper to epoxyglass laminate)                    α.sub.z = Heat expansion coefficient on Z axis                          P(t) = Surface profile                                                        D stab = Dimensional stability                                           

The parameters Tg, VO, C_(u) are influenced mainly by thecharacteristics of the resin used in the laminates, while the parametersε, α_(z), P(t) and D stab are directly influenced by the characteristicsof the fabrics.

Since the boards used for the production of printed circuitsincreasingly take the form of products intended for high-precisionengineering it is of fundamental importance to improve the parameters ofthe second category, i.e. obtain perfect "monolithicity" of the resinwith the glass fabric.

For example, high values of the heat expansion coefficient on the Zaxis, α_(z), cannot be tolerated, because the latter, during thechemical and heat processes to for processing of the laminate, may giverise deformations of the printed circuit board, making difficult toapply the electrical components onto the board itself. Moreover, thecomponents themselves, once fixed and operative, dissipate aconsiderable quantity of thermal energy, causing non-uniform thermalconditions in various points on the board which, at high values of α_(z)may cause serious problems of "resistance" of the epoxy glass laminate.

The situation becomes extremely delicate when the boards are formed bymulti-layer laminates. In this case the value α_(z), becomes of extremeimportance.

Poor dimensional stability, for example owing to non-uniformimpregnation of the fabric by the resin or owing to undesireddisplacements of the yarns inside the fabrics, or also owing to internaltension of the laminate, may adversely affect the electrical andmechanical characteristics of the printed circuit board.

It should be remembered in this connection that the copper tracks on avery fine line board have a width of 60-70 μm and are located at about100 μm from each other. Clearly in these conditions a slight localdeformation may compromise the integrity of the tracks or the insulationbetween them or adversely affect the values of the electrical andparasitic magnetic parameters.

It must also be noted that the configurations of the glass supportmedium may indirectly affect the parameters C_(u) and ε. If, forexample, the fabric is not suitable for promoting the resin impregnationprocess, small cavities may form in the laminate and falsify the valuesof ε and cause it to vary irregularly from zone to zone, as in the caseof a faulty dielectric of a capacitor. This causes a precarioussituation with regard to control of the disturbances during thetransmission of the signals (nowadays high-frequency) via the printedcircuit connections.

As regards the processability of the board, this is represented inparticular by its suitability for hole-boring. It is known that theholes are essential elements in a printed circuit board. The must besuitability metallized, i.e. the internal surface of the hole must becovered with a conductive metal, normally copper.

Producing laminates which can be easily processed ensures that properlymetallized holes are obtained. The fabrics produce using yarns with 0-6twists per meter would also ensure in this case a considerable increasein performance since they reduce substantially the intrinsicstrand-cutting resistance of the twisted yarns.

HPCT is an accelerated ageing test for the laminate which is firstimmersed for one hour in boiling water under pressure and then dippedinto a molten tin bath at 260° C. or at least 20 seconds there must beno blistering or "measling" on the laminate, thus indicating an adequatestrength of the bond between glass and resin. This is fundamental forensuring good results as regards the process involving preparation,assembly and soldering of the printed circuits. This strength depends toa large extent on the quality of impregnation of the fabric. Airpockets, referred to as "voids" (which occur mainly between filamentsinside an individual yarn), favour the penetration of moisture whichcauses delamination of the laminate. It would be expected that fabricsproduced using yarn with 0-6 twists per meter eliminate significantlythis phenomenon since they favour the maximum wettability of eachindividual filament (in these yarns the filament compression componentresulting from the tension of the twisted yarn is practicallynon-existent and a yarn with clearly separated filaments is obtained),thus resulting in optimum impregnation of the entire fabric.

Another parameter which is of increasing importance is that of theprofile of the surface of the laminates in the form of its twocomponents Roughness, R(t) and Wariness, W(t). The present values ofP(t) do not allow full exploitation of advanced techniques forproduction of the finished board such as surface mounting or even thatof direct application of the copper tracks onto the laminate. Theevenness of the laminate is therefore decisive in view of theincreasingly high frequencies of use to which the laminates will besubjected. In order to achieve these performance features, currenttechnology is based above all on reduction in the size of the tracks andthe distances between the various components "applied" onto the laminate(i.e. an increase in the density of the circuits ). The result onlaminates is that not only must the tracks be made narrower (60-70 μm),but their thickness must also be reduced on account of the "aspectratio" (less than 10 μm). It is obvious that such small dimensions ofthe copper tracks require a perfect adherence to the laminate which canonly be guaranteed by optimum evenness.

In addition to the technical parameters of the laminates for printedcircuits, it is necessary to take into account the price, which has asignificant impact on the cost of finished products for massconsumption. The price must therefore be kept low, while improving thetechnical characteristics of the laminates themselves.

In the industrial sector fabrics which use zero-twist yarns are alreadyknown. This yarn is known as "roving" and has micron diameter values andyarn counts much higher than those to which the present inventionrefers.

The high weight in grams provides these yarns with the strengthnecessary for them to be produced and hence woven without majorproblems. However, the situation is different as regards those fabricswith industrial uses which are manufactured using yarns associated withelectronics applications, i.e. yarns from 5.5 to 136 Tex and withdiameter values of 5 to 9 microns. In fact, as with fabrics intended forthe electronics sector, the are currently produced using yarns with 28to 40 twisting turns; the intrinsic weakness of the yarns of this weighthitherto has not permitted manufacture with zero twisting turns norweaving.

The present invention, as regards uses in the industrial sectortherefore refers solely to those fabrics which are currently produceusing yarns with 28 to 40 twisting turns and with a yarn count ofbetween 5.5 and 136 Tex.

These fabrics, if produced with yarns having the same gram weight andwith zero twists, would have an improved reinforcing function, i.e. themost important function which is required of glass fabrics in theseapplications.

It is in fact known to the person skilled in the art that the rigiditycoefficient of glass decreases with an increase in the number of twists.Furthermore, the improved intrinsic wettability of fabrics with zerotwists makes it possible to obtain, for the same overall weight, apreimpregnated product with greater glass content, thus improving themechanical and reinforcing properties of the preimpregnated productitself. In those applications where the thickness is a characteristic ofprimary importance (cf. the reinforcements in adhesive tapes), a moreeven fabric, such as that manufactured using zero-twist yarns, reducesthe risk of tearing of cardboard-box sealing tape following impact withother surfaces owing to its extremely limited excess thickness.

Moreover, considerations of an economic nature are also of extremeimportance; the glass fabric manufactured with zero-twist yarns offersconsiderable economic advantages compared to conventional glass fabric.

Generally in industrial applications where the mechanicalcharacteristics and the cost are of primary importance, the glassfabrics forming the subject of this invention represent a considerablecompetitive advantage in terms of both quality and cost.

Yarn Manufacturing Process

The glass fabric used for the aforementioned purposes is produced byprocessing E-glass (borosilicate) yarns.

The technique for the spinning of textile E-glass fibres is performed asfollows: a composition of selected raw materials (borosilicate) is fedinto a melting furnace which, heated to 1700° C., transforms the sandinto a fluid product. This homogeneous fluid is made to flow above thespinneret.

The spinneret is a meal plate made from platinum/rhodium alloys with alot of holes referred to as nozzles. The spinneret is heated by theJoule effect, being an integral part of an electric circuit. At atemperature of about 1100° C. (under the holes in suitable conditions),the liquid emerges from the holes in the plate and forms as manydroplets as there are openings (50, 100, 200 or 400 in number). Eachdroplet, upon reaching a certain size, separates from the metal forminga strand which, if it has a sufficient mass, continues to exert apulling force on the liquid which emerges from the holes, giving rise toa continuous strand. For the same (mass) flow through the holes andconstant speed of formation of the strand, the filaments will all havethe same diameter. Immediately underneath the spinneret, the filamentsare cooled by means of water jets. Subsequently the cooled and hardenedfibres undergo sizing and finally are wound onto the sleeves of the yarnstorage machines. The size consists of a mixture made up of: starch,lubricating products, anti-static and anti-mould agents. The storingmachine (winder) consists of a spindle rotating at high speed and a"zig-zagging" device for cross-winding the turns which are wound ontothe plastic sleeves specially mounted on the spindle.

Z or S Twisted Yarns20-40 pm

The yarn wound onto plastic sleeves (or "cakes"), after drying, isconveyed to the twisting department. The operation consists in unwindingthe yarn from the sleeve and rewinding it onto plastic tubes. The tubes,rotating at high speed and by means of small rings, apply a twistingaction to the glass yarn. The tube with the twisted yarn is ready forsale and will be subsequently used for the standard weaving operations.

The use of the first glass yarns with standard weaving machines (1950)proved difficult on account of the fragility of the yarn and the limitedprotection provided by the first rudimentary sizes.

It was therefore considered essential to use double twisted yarn. Firstthe single yarn was twisted with 160 Z turns and then combined so as toform double yarn, providing an opposite twist (called S) with 150 turnsfor correct balancing.

The yarn thus produced stood up well to all the subsequent processingoperations, but its cost was very high.

With the subsequent modifications made to textile machines, the use ofceramic yarn-guides and hard layered chromium plating on all the yarntravel paths, as well as the marked improvement in spinning sizes, itbecame possible to process single-twist yarns. The first single twistsconsisted in 40 Z turns (1965) which were further reduced to 28 Z turns(1975 to the present day).

At the same time, fabric impregnation improved considerably, allowinginitially better penetration of the filaments by the bridging agent(fibreglass resin) and subsequently better bonding of the resin with hemost exposed surface of the glass.

The range of printed circuits can be divided into he following types:rigid and multilayer. The standard rigid laminates have a thickness of1.55 mm and are manufactured with 8 layers of 7628 fabric weighing201-203 g/m². The number 7628 is a code defined by the IPC(Interconnecting and Packaging Electronic Circuits) standards and, inthis case, indicates a glass fabric formed by yarn with a count of 68Tex (68 g/1,000 m) with 12.6 weft yarns per cm and 17 warp yarns per cm.At present the yarn used for this fabric has 28 twisting turns per meteralong the weft and warp and is formed by 9-micron filaments.

The multilayer laminates have a smaller thickness and thus become lessrigid. The thicknesses range from 0.05 to 0.8 mm.

The fabrics used are of the following types (IPC designation): 1080,2112, 2116, 2113, 2313, 1675, 2125 and 2165.

This entire range of fabrics is produced using yarns with 40 twistingturns per meter (both along the weft and along the warp) and the yarncounts are: 5.5, 11, 22 or 34 Tex with 5, 6 or 7-micron filaments.

The heights of the fabrics are an important variant and range from 96.5to 132cm (initial use is being made of 142 cm). At present there arevery wide tolerances as regards the various parameters which define thefabric.

On account of the increasingly more stringent specifications of printedcircuits, the person skilled in the art is now forced to consider ascritical certain aspects of the laminates, such as the surface wariness,the dimensional stability, the warpage, the suitability for hole-boring,the thickness, the dielectric constant, etc., so that it can be saidthat the characteristics of the laminate depend not only on the resinsused, but also on the structure of the glass fabric.

Glass fabric, therefore, from being a simple support medium, has nowbecome a fundamental component which is essential for the quality of theproduct. This fabric, as well as being influenced by the characteristicsof the glass fibres described above, is also influenced by technologicalmanufacturing factors.

As is known to the person skilled in the art, glass fabric is formed byan interwining arrangement or woven framework, comprising a warp and aweft.

Considering the structure of the woven network, the characteristics ofthe glass fabric directly affect the surface profile of the fibreglassresin laminate P(t), which is defined as:

    P=R(t)+W(t)

where: R(t) is the maximum surface roughness due mainly to the copperand W(t) is the maximum underlying wariness due mainly to the supportmedium.

The parameter W(t) is a direct function of the glass fabric both asregards the type of yarn and intertwining of the weft with the warp.

The number of twists (per meter) of the yarn plays an important role indetermining the amplitude of W(t).

A further anomaly of the fabric which causes difficulty as regards itsuse in laminates is the lack of uniformity in the diameters of its basicfilaments, the negative influence of which is drastically increased athigh twisting turns (25-40 turns per meter) and with inconsistency oftwists per associated unit of measurement of the twisting system (moreor less 25% with respect to the number of turns between start and end ofreel). It cab cause lack of uniformity in the thickness of the fabric,problems of dimensional stability of the laminate owing to the presenceof abnormal internal stresses and finally critical conditions during thehole boring process.

Finally a dangerous defect of the glass fabric consists in imperfectcleanliness owing to the presence of residues of organic compounds, suchas starch, used in the weaving process and later removed, as explainedbelow. Their presence in the laminate alters its chemical and physicalproperties. It is obvious that the twisted filaments are very compactand hence more difficult to clean owing to the difficulty with whichheat is able to penetrate them and consequently owing to the difficultywith which the distillation vapours of the size present are able toescape.

Weaving is the basic operation for conversion of the yarn into fabric.

Transformation of the yarn into fabric is performed by means of thefollowing working phases:

warping: operation which consists in transferring the glass yarns fromthe reels to the beams, keeping them taut and parallel;

gluing or sizing: operation by means of which the glass yarns areprotected and lubricated with organic polymers, called dressings orsizes in order to make them suitable for the weaving process;

weaving: operation performed on weaving machines which iterwine groupsof yarns perpendicular to each other; those which run vertically (at 0°)form the warp and those arranged at 90° form the weft; the intertwiningarrangement of the yarns is known as the "weave";

finishing: which comprises the operations of:

de-sizing: performed thermally, during which the sizes are removed fromthe glass fabric treatment with surface agents: consisting in a chemicalreaction between the surface of the glass and bonding agents, so thatthe fabric is made reactive towards resins in general.

The conventional technology now used in the manufacture of glass fabricinvolves the use of air looms, in which the weft yarn is blown towardsthe warp.

In air looms the yarn blown by the air jet must have a consistency whichis markedly superior to that required for mechanical transportation, sothat the tendency has been to use yarns with a high number of twists andan abundant amount of size.

This technological aspect conflicts with the possibility of producingzero-twist glass fabrics suitable for use in the manufacture of thelatest generation printed circuits.

The Applicant, on the other hand, has made the particular choice ofusing rapier looms for the production of glass fabrics. Rapier looms arecommonly used in the manufacture of textile products made of natural orsynthetic fibre.

In order to use rapier loom in the manufacture, of fibreglass fabrics,the Applicant has developed several original improvements. In thisconnection reference is made to EP 0477138 and EP 0477139.

SUMMARY OF THE INVENTION

It has now been discovered, and this discovery represents a subject ofthe present invention, that with a glass fabric consisting of a yarnwith zero-six twists it is possible to manufacture laminates for printedcircuits which satisfy the requisites arising from the progressiveminiaturization of said circuits, together with the implications thereoffrom a technological and qualitative point of view. This fabric, in apreferred embodiment of the invention, is made using yarn with 0-6twists per meter both along the weft and along the warp withproportionally less favourable results, it can also be made using yarnwith 0-6 twists per meter either exclusively along the warp orexclusively along the weft (-40% and -60%, respectively).

Furthermore, the present invention relates to the use of glass fabrics,consisting of yarns with 0-6 twists for industrial uses as,a reinforcingmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGS. 1A and 1B show profiles of a fabric according to the priorart, FIG. 1A with twisted yarns and according to the present invention,FIG. 1B with parallel yarns.

DETAILED DESCRIPTION OF THE INVENTION

The glass fabric according to the present invention is prepared from aglass yarn with zero twists in a preferred embodiment of the inventionthis yarn is used both along the warp and along the weft or withproportionally less favourable results only along the weft or only alongthe warp.

Yarn which has a number of twists per meter ranging from 0 to 6 isdefined as zero-twist yarn.

Zero-twist glass yarn is known and used in some, industrial sectors,where the yarn counts (from 160 to about 2400 Tex) and the diametervalues of the basic filaments (from 9 to 20 microns) of the yarns usedare extremely high such that, from a technical and production point ofview, they are considered to be a product which is altogether differentfrom yarn for electrical engineering and electronics applications, sincethey are also made of al type of glass not suitable for electronic use.

In the electrical engineering and electronics sectors, whereborosilicate glass (E) yarns are used and where, as said, the yarncounts (from 5.5 to 136 Tex) and the diameter values (from 5 to 9microns) are considerably lower and the qualitative and technologicalrequirements are somewhat more stringent, hitherto it has not beenpossible to manufacture fabrics using this yarn with zero twists, onaccount of its extremely delicate nature and the consequentimpossibility of processing it.

At present, there exist two methods for the production of yarns withzero twist turns:

1. The most simple solution consists in replacement of the twistingmachines with spooling machines. Instead of the classic system usingtubes which twist the yarn, it is necessary to adopt a system forrewinding the yarn/which does not impart any twisting turns. The sleeve,which receives the yarn underneath the spinneret is dried and thenconveyed away for spooling The spooler is a machine which, by causing aplastic spindle to rotate at high speed, rewinds the yarn from thesleeve without twisting. This operation is simple and economical in thatthe rewinding speed is higher than that of the twisting tubes and theinvestment outlay is of a more modest nature. The difficulties ariseduring the preparation of a good-quality textile yarn. In fact, glassyarn is extremely susceptible to wear and the filaments are easilybroken if they are not suitably protected. Twisting, as alreadydescribed above, performed an important role in holding together the50-400filaments and the only purpose of the standard size was to makethe yarn slide more easily. With zero-twist yarn the role of the sizebecomes of vital importance; it must ensure that defects do not ariseduring spinning by uniting the filaments of the yarn, protect the latterfrom friction and allow the yarn to remain soft and flexible. It shouldbe remembered that the flexibility of glass, which has a very highYoung's bending modulus, depends on the fact that it is composed of aset of basic filaments with a very small diameter (5-9 m). The reelwhich is delivered for the weaving operation must be very carefullyprepared and must not, under any circumstances, have imperfections,especially along the edges where breakages of the filaments may occurwith dramatic consequences.

2The most innovative solution is the production of the spool directlyunderneath the spinneret eliminating the subsequent operation ofspooling described in the previous solution. The problem, which iscurrently being definitively resolved, consists in obtaining a constantdiameter for the individual filaments during the phase of winding ontothe spool which has different dimensions from the normal spinningsleeves or "cakes". It is therefore necessary to use a newly designedwinding machine which has greater precision with regard to the automaticsystem for varying the number of turns of the spool so as to keep thewinding speed and the pulling force absolutely constant. The size, inthis case too, is of fundamental importance and its preparation musttake into account that the drying operation after spinning is performedon a spool with dimensions different from a standard sleeve or "cake"normally used. This solution is the preferred one of the presentinvention because it uses the spool as it is supplied from spinningwith. Out having to undergo handling during an additional operation(spooling) and therefore the zero-twist yarn thus produced will besubject to the first stresses only during the subsequent weavingoperations.

The weaving operation is performed on rapier looms. In a preferredembodiment of the invention, the loom uses the rapier described in theaforementioned European patent applications.

In said preferred embodiment of the invention, the rapier, or otherwisecalled gripper, is characterized by the fact that on the external sideof the lateral wall of the supporting gripper directed towards the tipof the shed there is provided a clamp for the weft yarn to be insertedinto the shed. In particular, the fabric of the present invention ischaracterized by double wettability, with half the roughness value. Letus now consider the schematic diagram in the accompanying figure showinga profile view of a fabric with a twisted yarn of basic filaments of 5μm, 40 twists, (1), and compare it with that of a fabric with yarn againconsisting of 5 μm filaments which are not twisted and parallel, (2),obtained according to the present invention.

It can be clearly seen that with yarns with a low number of or zerotwists W(t) is much thinner than in the case where there is a highnumber of twists. The profile of the laminate affects the quality ofphotographic reproduction of the image on the dry film for incision ofthe printed circuit.

Fabrics with such a low surface roughness also allow new revolutionarytechniques to be adopted in the production of printed circuit boards,which envisage the direct transferability of the copper etched on thelaminate, with enormous economic, ecological and technologicaladvantages, such as a drastic reduction in the amount of copper used,the elimination of the etching operation with consequent elimination ofthe refluent acids, and greater fineness of the printed circuit tracks.

The advantages for the laminates produced with the new fabric can besummarized as follows:

reduction of the heat expansion coefficient α_(z), which is now veryhigh; in fact α_(z) is reduced because the impregnated fabric is alreadyin its condition of maximum expansion due to opening of the fibresresulting from the use of non-twisted yarn;

surface panel roughness with very low values;

improved hole-boring characteristics of the laminate and better boringquality;

surface free from inclusions and excess thicknesses;

high dimensional stability D_(stab) since in glass fabric the yarns, notbeing twisted, are arranged so as to be perfectly aligned and, having ahigher tensile modulus (no loss as a result of twisting) are more stablein both directions (at 0° and at 90°);

smaller number of microscopic voids, due to improved impregnation of thefabrics with zero-turn yarns;

greater resistance to "measles effect".

The use of the new fabric also offers advantages for the laminatemanufacturing process. Since the new fabric has a shorter wettabiltytime, there is a saving in the amount of solvent in the epoxy resinmixture which may be more viscous; higher speed of impregnation an hencegreater production capacity the impregnation machine.

As regards fabrics intended for industrial reinforcing uses, producedwith zero-twist yarn, the advantages can be summarized as follows:

improved modulus of rigidity, improved wettability, resulting inimproved mechanical characteristics of the finished product;

reduced thickness and roughness for the same weight of the finishedproduct

lower cost.

The fabric according to the invention can be used for example as a basefor spreading with PVC resins, as support fabrics for fire-resistantresins or fabrics for marine applications, for reinforcing thermoplasticand thermosetting resins, as decorative fabrics or internal and externalmural fabrics, as well as for reinforcing paper or thermoplastic filmsand the like.

In a typical embodiment thereof, the fabric according to the presentinvention has the following characteristics:

    ______________________________________                                        Weight            g/m.sup.2   45-230                                          Thickness         mm          0.045-0.18                                      Tensile strength (warp)                                                                         kg/cm       6-26                                            Elongation upon breakage (warp)                                                                 %           1                                               Tensile strength (weft)                                                                         kg/cm       4-17                                            Elongation upon breakage (weft)                                                                 %           1.2                                             Loss on ignition  %           0.1-0.17                                        Wet-Through test  1/20 cm.sup.2 /60"                                                                        <5                                              Wet-Out test      min.        <2                                              Surface pile      strands/1000 m                                                                            <2000                                           Distortion        %           <1                                              Textile defects   %           <1                                              Loom performance  %           90                                              Loom speed        strokes/min.                                                                              500                                             Cost of yarn      % compared  95                                                                to Standard                                                 Twisting turns    turns/m     0-6                                             Fabric waviness, W.sub.t                                                                        % compared  50-60                                                             to Standard                                                 ______________________________________                                    

We claim:
 1. In a woven glass fabric for use as a reinforcement in aprinted circuit board and comprised of continuous glass filament warpyarns and weft yarns, the improvement wherein at least one of the warpyarns and the weft yarns is zero-twist yarns made of glass filamentshaving a diameter of between about 5 and 9 microns and a yarn count ofbetween about 5.5 and 136 Tex.
 2. Glass fabric according to claim 1,wherein the warp yarns and the weft yarns are composed of zero-twistyarn.
 3. Glass fabric according to claim 1, wherein only the weft yarnsare composed of zero-twist yarn.
 4. Glass fabric according to claim 1,wherein only the warp yarns are composed of zero-twist yarn.
 5. Fabricaccording to claim 1, wherein the glass filaments are borosilicate (E)glass.
 6. Fabric according to claim 2, characterized in that it has thefollowing characteristics;

    ______________________________________                                        Weight          g/m.sup.2                                                                             45-230                                                Thickness       mm     0.045-0.18                                             ______________________________________                                    


7. Printed circuits comprising a laminate containing the glass fabricaccording to claim
 1. 8. Process for manufacturing the fabric accord toclaim 1, comprising weaving the fabric in a gripper loom.
 9. Processaccording to claim 8, wherein the gripper loom has on an external sideof a lateral wall directed towards a shed, a clamp for the weft yarns tobe inserted into the shed.